Air screw for aircraft



Feb. 14,1939. 'A; H R, FEDDEN ET A AIR SCREW FOR AIRCRAFT Filed April '14, 1938 2 Sheets-Sheet 1 l I llnl lllllllllllllllllllli||ll||l|||l| Feb. 1 4, 1939.

A H R FEDDEN ET AL 2,147,336

AIR Scmsw FOR AIRCRAFT Filed April 14, 1938 2 Street s-Sheet 2' Patented Feb. 14,1939

UNITED STATES PATENT OFFICE 2,101,356 AIR scanw son. Amcm'r Alfred Hubert Roy Fedden and nan-y Lawley Milner, Bristol, England, assignors to Rotol Airscrews limited, British company I Gloucester, England, a

Application April 14,1938, Serial 'No, 202,058 a x In Great Britain April 22, 1937.

5 ter ial such as magnesium alloy the root of the blade has to be reinforced by a surrounding sleeve or cap of harder material such as steel. The stress 8 between these two parts tends to cause fretting 3 between them. The object of this invention is to prevent such fretting.

According to one feature of the invention the internal diameter of the surrounding sleeve is outwardly tapered at its outer end to receive a tapered bush between the sleeve and the bladeroot, the angles of the said tapers being so small that, after having been forced in under high pressure, the bush is retained by friction against withdrawal. I

, According to another feature of the invention, the angle of taper of the bush is slightly greater than the angle of taper of the space between the sleeve and the blade-root so that, as the bush is forced in, the sleeve is expanded to a greater extent at,its outer end than elsewhere.

According to another feature of the invention a method of removing the bush consists in forcing fluid under high pressure into theinterior of the sleeve. In this way the sleeve is expanded whereby the bush can be withdrawn. 1

A specific embodiment of the invention will now be describedby way of example with reference to the accompanyingdrawings, in which:

Figure 1 is an elevation of the root" portion of an air screw blade showing, in section, the reinv forcing sleeve and other associated parts; Figure 2 is asimilar view, to an enlarged scale, of the outer endof the reinforcing sleeve; and

Figure 3.1san elevation on the line 3-3 of Figure 2? As shown in the drawings, the air screwblade i0 which is of magnesium alloy or aluminum alluy or other -comparatively soft metal, is formed at its root end-with screw-threads ll of rounded V. form. (Jo-operating screw-threads are formed on the interior of a steel sleeve I! which is screwed on to the blade-root, but is not shrunk on. .The sleeve l2 isformed with a longoutwardly-extending socket members i3 having external buttress threads ii for engagement by a nut IS. The nut has an inwardly-directed flange iii to engage the outr end of a bush I 1 which is wedged-shaped in a direction longitudinally of the blade-axis.

The sleeve L2 engages an inner thrust bearing (not shown) and an outer ball. bearing l8 which is supported in a socket portion 19 of the 4 Claims. (01. 170-113 air screw hub. Sealing means 20, Ii, 22, are provided for preventing the escape of lubricant'from the bearings surrounding the sleeve l2. j' The shape of the bush i1 is shown more clearly in Figure 2. Its internal face is cylindrical and 5 I parallel with the plain portion of the blade-root. Its outer surface is of generallyconical form so that, as seen in Figure 2, the bush is wedgedshaped, having an angle of taper of about 6".

The internal surface of the surrounding socket 10 member i3 is of similar conicalshape but has an angle of taper. when in its unstressed condition, of somewhat less, say about 5.

When the parts are to be assembled, the nut i5 is passed along the blade from the root end and is followed by the bush ii. The blade-root is v now screwed into the sleeve I 2 until it assumes the position shown 'in Figure 1. The bush i1 is now-pressed down into the socket member I! of the sleeve, the nut i5' is engaged with the threads i4 and tightened. As the nut is tightened its flange i6 engages the outer end of the bush and forces the bush down between the blade i0 and the socket member ii. The result of'this operation is that the socket member is is plasti cally expanded until the bush makes'cohtact with ,the sleeve at both its upper and lower ends. Owing to the difference in the angle of taper between the external surface or the bush and the internal surface of the-socket member, the sleeve will be expanded to a greater extent at its outer end than 'at its inner end. In Figure 2 the chain, lines show the socket member i3 in the position it occupies prior to tightening the nut i5 andthe full lines show it in its expanded condition.

Owing to the small angle oftaper ofthe 'bush, the friction between the bush and the socket member and between the bush and the blade re= tains the bush in the position shown in Figure 2, so that it is impossible to withdraw the bush ex- 40 cent by the special method hereinafter described.

The bush I'I may be formed of a suitably rein- L forced phenol-formaldehyde condensation product such as that sold under the registered trade-mark Bakelite. Alternatively, especially where the-4 blade is of an alluminium alloy, the bush could be of a metal such as aluminium; The pressure'between the sleeve and the bush and between the bush and the blade may be of the order of from 2000 to 4000 pounds per square inch.

The nut i5 is locked by'means of a washer 23 .having a tooth 24 and flats 25 to engage'serrations 28 on the longitudinally-extending part I of the nut. The washer 23 is held on the, sleeve I2 by means of a stud 21 screw-threaded into a hole 28 in the socket member.

' To remove the blade l0 from the sleeve 12 the stud 21 is removed and the nut I5 unscrewed. The nipple of a grease gim is now screwed into the hole 28 and grease is pumped in under high pressure. Owing to the greater pressure between the blade, bush and socket member at the outer end of the socket member, the grease first works inwardly without escaping from the socket member. The pressure of the grease expands the sleeve it away from the bush first'at the inner end of the bush and then outwardly towards the outer end of the sleeve, until finally the surfaces are completely parted.v The blade may now be easily unscrewed from the sleeve.

To facilitate the entry of the-grease between the bush and the socket member IS the bush may be formed with a parallel portion 29 running round it which thus leaves a cavity between it and the socket member l3.

Although only one of the studs 21 is shown, a plurality of holes 28 could be provided around the socket member l3 to all of which fluid pressure could be applied simultaneously.

Although the primary object of the invention is to eliminate'fretting between the air screw blade and the surrounding steel sleeve, it has also the advantage that the sleeve 12 can be removed from the blade whereby the parts may be inspected from time to time.

We claim:

1. An air screw comprising an air screw blade having a root-end, a metal sleeve surrounding the root-end and forming at its outer end a tapering space between it and the root-end, and

a tapering bush adapted to be forced into the said space, wherein the taper of the bush is 'device\ carried by said stud-like member and greater than that of the said space and wherein the sleeve is formed between the inner and outer ends of the bush with an orifice through which fluid under pressure may be admitted to expand the sleeve away from the blade-root.

2, In an air screw, an air screw-blade having a root-end, a metal sleeve surrounding the rootend, a tapering bush adapted to be forced into the outer end of the sleeve between the sleeve and the root-end, a nut screw-threaded to the outer end of the sleeve, an inwardly-directed flange carried by the nut to engage the outer end of the bush whereby tightening of the nut forces the bush into the sleeve, external serrations formed on the nut, a stud-like member engaging a screw-threaded hole passing entirely through the said sleeve at a point between the inner and outer ends of the said bush, and a locking engaging the said serrations.

3. An air screw blade comprising a root-end,

a metal sleeve surrounding the root-end so as to leave a tapering space of about 5 and adapted to be elastically expanded, and a bush having a taper of about 6 and adapted to be forced ALFRED HUBERT ROY FEDDEN. HARRY LAWLEY MEIER. 

